The water quality control for high-pressure steam boilers | tiwater.info
Almost completely desalinated water is required to be used for feeding steam boilers with the pressure of more than 14 MPa. The salt content of feed water in terms of Na should be no more than 50 mcg/kg, and for direct-flow boilers no more than 20 mcg/kg. The pH value of such water should be equal to 9.1+ – 0.1. At this pH value, the presence of free carbon dioxide in the feed water is completely excluded. However, the feed water may contain bound carbon dioxide in the form of carbonate and bicarbonate which will pass into steam in the boiler and cause corrosion of the steam condensate tract.
For high-pressure boilers, it is customary to use a water treatment scheme with H-OH filters. I.e., all dissolved ions are removed from the water. In this case, carbon dioxide is also removed by the OH-filter. However, the carbon dioxide in the feed water can come from condensate.
To bind carbon dioxide in the condensate and prevent carbon dioxide corrosion, ammonia water is dosed into the feed water. Ammonia reacts with water to form ammonium hydrate (NH4OH). This increases the pH and electrical conductivity of the feed water. In the boiler, the ammonia evaporates with steam. Carbon dioxide with steam is also removed from the water. When steam condenses, the ammonia in the condensate passes into ammonium hydrate and binds carbon dioxide to bicarbonate according to equation (1). If there is more ammonium hydrate than carbon dioxide, then some of the carbon dioxide will be bound into ammonium carbonate.
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